1. Field of the Invention
The present invention relates generally to the field of telephone communications and more specifically to a method, system and computer program product for improved narrow frequency band signal detection for echo cancellation in telephone communication systems. The present invention includes use of various technologies described in the references identified in the appended LIST OF REFERENCES and cross-referenced throughout the specification by numerals in brackets corresponding to the respective references, the entire contents of which is incorporated herein by reference.
2. Discussion of the Background
The late 1950s marked the birth of echo control in the telecommunications industry with the development of the first echo-suppression devices. These systems, first employed to manage echo generated primarily in satellite circuits, were essentially voice-activated switches that transmitted a voice path and then turned off to block any echo signal. Although echo suppressers reduced echo caused by transmission problems in the network, they also resulted in choppy first syllables and artificial volume adjustment. In addition, they eliminated double-talk capabilities, greatly reducing the ability to achieve natural conversations. [1]
Echo-cancellation theory was developed in the early 1960s by ATandT Bell Labs, followed by the introduction of the first echo-cancellation system in the late 1960s by COMSAT TeleSystems (previously a division of COMSAT Laboratories). COMSAT designed the first analog echo canceller systems to demonstrate the feasibility and performance of satellite communications networks. Based on analog processes, these early echo-cancellation systems were implemented across satellite communications networks to demonstrate the network""s performance for long-distance, cross-continental telephony. These systems were not commercially viable, however, because of their size and manufacturing costs. [1]
In the late 1970s, COMSAT TeleSystems developed and sold the first commercial analog echo cancellers, which were mainly digital devices with an analog interface to the network. The semiconductor revolution of the early 1980s marked the switch from analog to digital telecommunications networks. More sophisticated digital interface, multi-channel echo-canceller systems were also developed to address new echo problems associated with long-distance digital telephony systems. Based on application-specific integrated circuit (ASIC) technology, these new echo cancellers utilized high-speed digital signal-processing techniques to model and subtract the echo from the echo return path. The result was a new digital echo-cancellation technique that outperformed existing suppression-based techniques, creating improved network performance. [1]
The 1990s have witnessed explosive growth in the wireless telecommunications industry, resulting from deregulation that has brought to market new analog and digital wireless handsets, numerous network carriers, and new digital network infrastructures such as TDMA, CDMA, and GSM. According to the Cellular Telecommunications Industry Association (CTIA), new subscribers are driving the growth of the wireless market at an annual rate of 40 percent. With wireless telephony being widely implemented and competition increasing as new wireless carriers enter the market, superior voice transmission quality and customer service have now become key determining factors for subscribers evaluating a carrier""s network. Understanding and overcoming the inherent echo problems associated with such digital telecommunications networks will enable network operators and telephone companies to offer subscribers the network performance and voice quality they are demanding today. [1]
The detection of the presence of narrow frequency band signals in telephony circuits is of importance for the operation of voice based echo cancellation algorithms. The performance of the voice based echo cancellation algorithms typically degrade upon processing of such narrow frequency band signals. In addition, a control algorithm for the echo cancellation typically malfunctions upon processing of such narrow frequency band signals. The echo cancellation control algorithm uses narrow frequency band signal detection to schedule an adaptive echo filter update and to perform an echo return loss computation.
The above-noted conventional methods and systems typically use spectral analysis techniques in order to detect the presence of single and multiple frequency tonal signals. Some algorithms use frequency domain transforms, such as the Fast Fourier Transform (FFT), to convert samples into the frequency domain and then execute decision logic to identify tones. However, such transforms typically require extensive computations, resulting in the consumption of substantial system resources.
Other conventional systems and methods use a bank of limited bandwidth filters in the time domain to detect certain frequencies. However, such systems and methods typically require multiple filters to detect the presence of any pre-defined tonal signals, resulting in increased system complexity and requiring system resources proportional to a total number of taps of all of the filters.
In addition, the above-noted methods and systems typically only detect tonal signals, which generally is not sufficient for use in an echo canceller control unit.
Accordingly, an object of this invention is to provide a novel method, system and computer program product for improved narrow band signal detection based on time domain analysis.
Another object of this invention is to provide a novel method, system and computer program product for detection of narrow frequency band digitized signals in telephony circuits.
Another object of this invention is to provide a novel method, system and computer program product for detection of narrow frequency band digitized signals for echo cancellation in telephony circuits.
Another object of this invention is to provide a novel method, system and computer program product for detection of narrow frequency band digitized signals in telephony circuits using zero crossing and amplitude analysis techniques.
Another object of this invention is to provide a novel method, system and computer program product for detection of narrow frequency band digitized signals for use in satellite-based digital telephony and data communication network systems.
The above and other objects are achieved according to the present invention by providing a novel digital signal detection system, method and computer program product, including (a) partitioning a digital signal into a plurality of frames; (b) calculating a number of zero crossings of the digital signal within each frame; (c) determining whether or not an absolute value of a difference of the number of zero crossings in between frames is less than a first predetermined value; (d) determining whether or not a sum of the zero crossings of the frames is greater than or equal to a second predetermined value and less than or equal to a third predetermined value; (e) calculating a maximum amplitude of the digital signal for each of the frames; (f) calculating an average amplitude of the digital signal for all of the frames; (g) determining whether or not an absolute value of a difference between the maximum amplitude of a frame and the average amplitude is less than a fourth predetermined value times the average amplitude for each of the frames; and (h) determining that a frame contains a narrowband digital signal if the determining steps (c), (d), and (g) are met.
Advantageously, the present invention not only detects single frequency tones but also narrow frequency band signals that are not necessarily tonal signals. In addition, since the present invention operates in the time domain, the present invention requires limited system resources, and uses simple decision logic, so that a fast detection can be achieved, as compared to conventional frequency domain detection techniques.